We have used x-ray small-angle solution scattering to investigate the conformation of several intact myosin S1 kinetic intermediates. The structure of a smaller portion of S1 (called S1Dc) complexed with ADP BeFx, ADP AlF4 and ADP Vi analogs has been determined, and we used S1 + analogs and ATP (actively hydrolyzing S1) to selectively populate these intermediates. The results we obtained are correlated with the S1 and S1Dc crystallographic results. We find that there is a unitary large scale conformational change in S1 (which seems to occur just before or during ATP hydrolysis) which acts to cock the acto-myosin system for subsequent energy transduction. Modeling the observed changes in scattering curves using F-actin and S1 structures indicates that the longitudinal projection of the force-producing portion of the working stroke is less than 6 nm. This work has recently been published in the Journal of Molecular Biology. This year we studied more kinetic intermediates, using new ATP analogues.